1. Field of the Invention
The present invention is directed to removing CO2 from CH4-containing gases. In particular, the present invention is directed to scrubbing CO2 from a CH4-containing gas using an aqueous stream, forming a CO2-reduced CH4-containing gas and a CO2-enriched aqueous stream, processing the CO2-reduced CH4-containing gas to form salable liquid products, and disposing of the CO2-enriched aqueous stream.
2. Description of the Related Art
The conversion of remote natural gas assets into transportation fuels has become more desirable because of the need to exploit existing natural gas assets as a way to satisfy the increasing need for transportation fuels. Generally, the term xe2x80x9cremote natural gasxe2x80x9d refers to a natural gas asset that cannot be economically shipped to a commercial market by pipeline.
Conventionally, two approaches exist for converting remote natural gases into conventional transportation fuels and lubricants including, but not limited to, gasoline, diesel fuel, jet fuel, lube base stocks, and the like. The first approach comprises converting natural gas into synthesis gas by partial oxidation, followed by a Fischer-Tropsch process, and further refining of resulting Fischer-Tropsch products. The second approach comprises converting natural gas into synthesis gas by partial oxidation, followed by methanol synthesis wherein the synthesized methanol is subsequently converted into highly aromatic gasoline by a Methanol-To-Gasoline (MTG) process. Both of these approaches use synthesis gas as an intermediate. Also, while other approaches exist for using natural gas in remote locations, such approaches do not produce conventional transportation fuels and lubricants, but instead produce other petroleum products including, but not limited to, liquified natural gas (LNG) and converted methanol. The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into salable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having low crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels.
Catalysts and conditions for performing Fischer-Tropsch reactions are well known to those of skill in the art, and are described, for example, in EP 0 921 184A1, the contents of which are hereby incorporated by reference in their entirety. A schematic of a conventional Fischer-Tropsch process is shown in FIG. 1.
The Fischer-Tropsch process can be understood by examining the stoichometry of the reaction that occurs during a Fischer-Tropsch process. For example, during Fischer-Tropsch processing, synthesis gas (i.e., a mixture including CO2 and hydrogen), is generated, typically from at least one of three basic reactions. Typical Fischer-Tropsch reaction products include paraffins and olefins, generally represented by the formula nCH2. While this formula accurately defines mono-olefin products, it only approximately defines C5+ paraffin products. The value of n (i.e., the average carbon number of the product) is determined by reaction conditions including, but not limited to, temperature, pressure, space rate, catalyst type and synthesis gas composition. The desired net synthesis gas stoichiometry for a Fischer-Tropsch reaction is independent of the average carbon number (n) of the product and is about 2.0, as determined by the following reaction equation:
nCO+2nH2xe2x86x92nH2O+nCH2
where nCH2 represents typical Fischer-Tropsch reaction products such as, for example, olefins and paraffins.
The three general reactions that produce synthesis gas from methane are as follows:
1. steam reforming of methane: CH4+H2Oxe2x86x92CO+3H2;
2. dry reforming, or reaction between CO2 and methane: CH4+CO2xe2x86x922CO+2H2; and
3. partial oxidation using oxygen: CH4+1/2O2xe2x86x92CO+2H2.
Although the above general reactions are the basic reactions used to produce synthesis gas, the ratio of hydrogen to carbon monoxide produced by the above reactions is not always adequate for the desired Fischer-Tropsch conversion ratio of 2.0. For example, in the steam reforming reaction, the resulting ratio of hydrogen to carbon monoxide is 3.0, which is higher than the desired hydrogen to carbon ratio of 2.0 for a Fischer-Tropsch conversion. Similarly, in the dry reforming reaction, the resulting hydrogen to carbon monoxide ratio is 1.0, which is lower than the desired hydrogen to carbon monoxide ratio of 2.0 for a Fischer-Tropsch conversion. In addition to exhibiting a hydrogen to carbon monoxide ratio that is lower than the desired ratio for a Fischer-Tropsch conversion, the above dry reforming reaction also suffers from problems associated with rapid carbon deposition. Finally, because the above partial oxidation reaction provides a hydrogen to carbon monoxide ratio of 2.0, the partial oxidation reaction is the preferred reaction for Fischer-Tropsch conversions.
In commercial practice, an amount of steam added to a partial oxidation reformer can control carbon formation. Likewise, certain amounts of CO2 can be tolerated in the feed. Thus, even though partial oxidation is the preferred reaction for Fischer-Tropsch conversions, all of the above reactions can occur, to some extent, in an oxidation reformer.
During partial oxidation, CO2 forms because the reaction is not perfectly selective. That is, some amount of methane in the reaction will react with oxygen to form CO2 by complete combustion. The reaction of methane with oxygen to form CO2 is generally represented by the following reactions:
CH4+O2xe2x86x92CO2+2H2
and
CH4+2O2xe2x86x92CO2+2H2O.
Furthermore, steam added to the reformer to control coking, or steam produced during the Fischer-Tropsch reaction can react with CO to form CO2 in a water gas shift reaction represented by the following general reaction:
CO+H2Oxe2x86x92CO2+H2.
Thus, invariably a significant amount of CO2 is formed during the conversion of methane into transportation fuels and lubricants by the Fischer-Tropsch process. The CO2 produced during the Fischer-Tropsch process exits the Fischer-Tropsch/Gas-To-Liquid (GTL) process in a tail gas exiting a Fischer-Tropsch facility. Tail gases exiting a Fischer-Tropsch/GTL process comprise any gases that remain unconsumed by the Fischer-Tropsch process.
The above equations represent general stoichiometric equations, they do not reflect an optimum synthesis gas composition for the kinetics or selectivity of a Fischer-Tropsch reaction. Moreover, depending on the nature of the Fischer-Tropsch catalyst, synthesis gas ratios other than about 2.0, typically less than about 2.0, are used to prepare the feed to a Fischer-Tropsch facility. However, because Fischer-Tropsch facilities typically produce products exhibiting a hydrogen to carbon ratio of about 2.0, the limiting reagent, typically H2, is consumed first. The extra reagent, typically CO, is then recycled back to the Fischer-Tropsch facility for further conversion. Synthesis gas compositions having hydrogen to carbon ratios other than about 2.0 are typically generated by recycling unused reagents.
As a result, there is an urgent need for a process that can reduce CO2 emissions from Fischer-Tropsch GTL processes, thereby minimizing adverse environmental effects that may be caused by such emissions. Furthermore, technology is needed to enable the processing of CO2-rich natural gas without the emission of the CO2 associated with the gas into the environment.
The present invention satisfies the above objectives by providing a process that removes CO2 from CH4-containing gases and isolates the removed CO2 from the environment by scrubbing with an aqueous stream. By scrubbing with an aqueous stream, the present invention avoids the need for costly CO2 isolation processes.
A method, according to the present invention, for removing CO2 from a gas stream can include contacting a gas stream, including methane and CO2, with an aqueous stream so that at least a portion of the CO2 in the gas stream is dissolved into the aqueous stream, creating a CO2-depleted gas stream having an enriched methane concentration, and a CO2-enriched aqueous stream. The CO2-enriched aqueous stream is then separated from the gas stream. Finally, the CO2-enriched aqueous stream is disposed of in, for example, at least one of a marine environment, a terrestrial formation or combination thereof.
A process of the present invention, for converting methane-containing gas in a Fischer-Tropsch GTL facility into liquid hydrocarbons can include contacting a methane-containing gas, being supplied to a Fischer-Tropsch GTL facility, with an aqueous stream so that at least a portion of CO2 in the methane-containing gas is dissolved into the aqueous stream creating a CO2-depleted methane-containing gas and a CO2-enriched aqueous stream. The CO2-enriched aqueous stream is then separated from the gas stream. Next, the CO2-enriched aqueous stream is disposed of in at least one of a marine environment, a terrestrial formation or combination thereof. Finally, the CO2-depleted methane-containing gas is processed in the Fischer-Tropsch GTL facility to obtain liquid hydrocarbons.
A method of the present invention for removing CO2 from a gas can include contacting a gas, including methane and CO2, at a pressure greater than about atmospheric pressure and less than a pressure of a source supplying the gas stream, with a CO2-selective adsorbent, creating a CO2-enriched adsorbent and a CO2-depleted gas having an enriched methane concentration. Next, the CO2-enriched adsorbent is treated to regenerate the adsorbent to be reused to contact the gas and to form a CO2 stream. The CO2 stream is then contacted with an aqueous stream so that at least a portion of the CO2 is dissolved into the aqueous stream, producing a CO2-enriched aqueous stream. Finally, the CO2-enriched aqueous stream is disposed of in at least one of a marine environment, a terrestrial formation or combination thereof.
In general, the present invention removes CO2 from CH4-containing gases, including gases being fed to Fischer-Tropsch GTL facilities by scrubbing CO2 from the gas using an aqueous stream, preferably at a pressure greater than about atmospheric pressure. Thus, one important advantage of the present invention is that it avoids having to use costly CO2 isolation processes including, but not limited to, CO2 compression, liquefaction or solidification to isolate CO2 from CH4-containing gases.